 It is one of the greatest mystery of science why large and complex animals appeared on Earth. 700 million years ago, there was a dramatic event, a so-called Snowball Earth event. Earth was frozen over for 50 million years and even at the equator, the ocean was frozen 2 km deep. Huge glaciers ground entire mountain sites to powder, releasing nutrients. And when the Snowball melted, these nutrients were moved with a meltwater into the ocean, creating an overkill of nutrients that changed Earth's ecosystems forever. Small microscopic bacteria were replaced by much larger eukaryotic algae. And these organisms are still microscopic, but they're a thousand times larger than a phototropic bacteria. And these organisms revolutionized the base of the food web. And without it, we would not be here today. To solve this mystery, we took ancient sedimentary rocks, we crushed them to powder, and we extracted molecules from them from ancient microorganisms. As an organic geochemist, part of my role was looking at these rocks from Central Australia and just removing the contaminants and looking at the real ancient molecules and finding out what that tells us about ancient life. These large creatures created the burst of energy that is needed for complex ecosystems out of which then animals can evolve. It was a revolution of ecosystems, it was the rise of algae.